1. Field of the Invention
This invention relates to a recording sheet conveying system of a pressure fixing type electrostatic printing apparatus, such as a facsimile terminal, copying apparatus, printer, etc., using a dry toner.
2. Description of the Prior Art
Nowadays, fixing of a facsimile print of the original copy transmitted by a facsimile terminal of the electrostatic printing type shows a tendency to rely more and more on pressure rather than heat and light, to perform the operation at increased speed and enhanced safety while conserving energy.
A pressure fixing type dry electrostatic printing apparatus includes a pair of pressing rollers through which a sheet having no image fixed thereto is passed, to obtain fixing of a toner image to the surface of the recording sheet.
No satisfactory results have been obtained with a pressure fixing system of the prior art in a copying apparatus and a facsimile terminal. This is largely because the pressure used for fixing an image is set at a low level to reduce the incidence of wrinkling and jamming of a recording sheet (which refer to the phenomenon of the copying sheet becoming wrinkled during conveying and making it impossible to continue conveying). The results achieved by pressure fixing can be increased by raising the pressure applied. However, an increase in pressure might render the pressure applied between the fixing rollers nonuniform, thereby causing irregularities in the fixed image and wrinkles in the sheet to be produced.
Thus the practice of fixing an image to a recording sheet in a pressure fixing system requires feeding of the recording sheet to the pressing rollers at an angle and with a posture that do not cause wrinkling, jamming and skewing (the phenomenon of sheet moving obliquely in the conveyed direction without moving straight ahead) of the sheet to occur.
Since the recording sheet fed between the pressing rollers has on its surface a toner image not yet fixed thereto, it is impossible for the operator to arbitrarily touch the sheet, so that difficulties are experienced to place the sheet in good condition to avoid wrinkling, jamming and skewing. Thus skewing of a sheet of a certain degree has been tolerated so long as formation of wrinkles can be avoided.
In the prior art of avoiding wrinkle formation, proposals have been made, as disclosed in Japanese Patent Publication No. 45802/77 and Japanese Utility Model Application Laid-open No. 38313/80, to provide a guide plate located in a position immediately before the recording sheet is introduced between the pair of fixing rollers in such a manner that opposite side edges of the guide plate are disposed on the line of contact of the pair of rollers, above it or below it, so as to avoid wrinkle formation in the sheet by virtue of the difference in the axial direction in the peripheral velocity between the rollers forming the pair.
However, with an increase in the speed of operation and better results achieved in fixing, the pressure at which the rollers force against each other increases and it has become difficult to positively avoid wrinkle formation merely by means of the guide plate. Moreover, difficulties have been experienced in correcting the skewing that inevitably occurs depending on the printing condition, such as when the recording sheet is white in its left half and black in its right half, for example. Phenomenons, such as wrinkling, jamming and skewing, show a tendency to grow once they occur.